The output torque of a permanent magnet synchronous motor (PMSM) (either a surface permanent magnet (SPM) or an interior permanent magnet (IPM) motor) may be determined by a voltage command and a phase advance angle. A specific output torque of the PMSM is determined by first selecting a specific quadrature axis (also referred to as the q-axis) reference current and a direct axis (also referred to as the d-axis) reference current, and then determining the voltage command and the phase advance angle based on the selected quadrature axis reference current and the direct axis reference current.
Electric Power Steering (EPS) systems use an electric motor (e.g., PMSM) to provide steering assist. When using a PMSM, Field Oriented Control (FOC) is utilized, which allows an alternating current (AC) poly-phase (e.g., three-phase) motor voltage and current signals to be transformed into a synchronously rotating reference frame, commonly referred to as the d-axis/q-axis reference frame, where the motor voltages and currents become direct current (DC) quantities. The FOC torque control technique is implemented either using feedforward methods of control or a closed-loop current feedback control, or some combination of them.
Application of closed-loop current control of PMSM to EPS systems has unique and demanding requirements outside of the control system's capability to track the desired assist torque command (i.e., motor torque command). Many of these requirements are associated with a balance of the torque response behavior, motor input disturbance characteristics, current measurement noise transmission characteristics, and robustness to the accuracy of the estimated electric motor parameter estimates. Consistency of performance throughout the operating range of the control system is desired, including operation throughout the motor velocity range and operation near the supply voltage limit. Unlike high voltage power applications utilizing PMSMs, the supply voltage available for the control system from a vehicle is not unlimited, and the motor used in these applications is typically sized as efficiently as possible to deliver steady state power requirements. This requires the current control to operate in a stable and predictable manner as the transient voltage available to the control system becomes smaller near the peak power point of PMSM operation. Therefore, the control system should be configured to operate as desired while requiring relatively small motor voltage command transients.